Diesel engine retarders, of the type sometimes called engine compression brakes, are used to slow down vehicles such as trucks, either without the application of the truck's normal wheel brakes or to enhance braking when used in cooperation with wheel brakes. In trucks which have such engine retarders, operation is generally as follows. First, fuel flow to the engine is shut off so as to stop the combustion process and subsequent power generation. Next, a device in the engine valve train opens the exhaust valve a slight amount at the end (top) of the usual compression stroke. As a result, the engine is turned into a very inefficient pump. The energy input to this pump, i.e. to the engine, comes from the inertia of the moving truck through the power train (transmission, axles, wheels, etc.). This pumping process (pump work) significantly slows the moving truck.
A typical compression-type brake can be understood by comparing it with a four-cycle engine that does not have a compression-type brake system. (It is noted, however, that most compression brake-type systems are useful on both two and four-cycle diesel engines.) Without a compression-type brake, on stroke 1, called the induction stroke, the piston moves down and an inlet valve opens. This draws air into the cylinder. If there is a turbo charger, the air is forced into the cylinder by boost pressure from the turbo charger. On stroke 2, called the compression stroke, the inlet valve closes and the piston moves up. The fuel mixture is thus compressed. The energy required to compress this air is produced by the driving wheels of the vehicle. On stroke 3, called the power stroke, fuel is injected into the cylinder, in turn igniting due to compression, forcing the piston back down the cylinder. As the piston is forced back down the cylinder, the energy is returned to the driving wheels. On stroke 4, called the exhaust stroke, the exhaust valve opens and the piston rises, pushing the exhaust gases out of the cylinder.
With a compression-type brake system, the typical four-cycle engine is modified from that described above. With a compression-type brake activated, on the compression stroke the inlet valve opens, and air is drawn or forced into the cylinder from the intake manifold. This is no different from the typical induction stroke. On the compression stroke, air is compressed to approximately 500 psi or higher by the engine piston. The energy required to compress the air is produced by the inertia of the truck's driving wheels. During the compression stroke, near top dead center, the compression-type brake opens the exhaust valves, venting the high pressure air and dissipating the stored energy through the exhaust system. In the power stroke, essentially no energy is returned to the piston, and thus, essentially no energy is returned to the driving wheels. There is a loss of energy. This loss is the engine retarding work done. During the exhaust stroke, the outlet valve opens and the piston rises, pushing the exhaust gases out of the cylinder. The exhaust stroke, during operation of a compression-type brake is no different than the exhaust stroke of a normal diesel engine.
Typically, trucks with engine retarders are provided with an overall on/off control switch in the truck cab. That is, the engine retarder is left "on" or "off" by the driver; and, when the retarder is "on" it will automatically engage when the driver takes pressure off the accelerator pedal or when pressure is applied to the wheel brakes, depending upon the system. Application of a compression brake-type engine retarder can produce as much or more power to stop the vehicle, than the engine can produce during normal operation. This is considered beneficial by truck operators in many instances, since it significantly reduces brake wear while still serving as an effective brake.
A major manufacturer of such engine retarders in the United States is Jacobs Vehicle Systems of Bloomfield, Connecticut. The systems manufactured by, or under the direction of, Jacobs Vehicle Systems, are generally available under the trademark "Jake Brake". At the present time, Jake Brake.RTM. Systems, or similar engine retarders, are found on many trucks, either installed by the manufacturer (for example, Freightliner, Peterbilt, Mack), or installed afterwards, by choice of the truck owner.
The use of such compression brake engine retarders, although considered highly effective for braking and safety, is associated with undesirable noise. In particular, compression brake operation is associated with a very distinctive, high amplitude, staccato noise or engine "bark". This noise is of a nature that cannot be adequately muffled, by conventional truck muffler systems. The noise is often so objectionable that in many municipalities, especially in hilly areas, signs are posted prohibiting the use of compression brake-type engine retarders.